This invention relates to a method and apparatus for performing handoff in mobile cellular systems such as, but not limited to code division multiple access (CDMA) systems.
In the field of wireless communication using mobile devices and plural spaced apart base stations, communication generally takes place between a mobile device and one of the base stations. As the mobile device moves among the plural base stations, the strength of the communication or power output the mobile device received at the first base station with which the mobile device is then communicating is measured. The strength of this signal will eventually diminish and the strength of the signal to a second of the plural base stations will increase. At some point the strength of the signal from the second base station will become stronger than the signal from the first base station whereupon communication between the mobile device and the second base station will commence and communication between the mobile device and the first base station will cease. The transfer is known in the art as xe2x80x9chandoffxe2x80x9d and the action is made in the network by coordinating the original base/mobile station and the new base/mobile station. In CDMA, the measurement of signal strength of the signal received at the mobile device can be made at the mobile device with the mobile device assisting in the handoff procedure.
When traveling away from a first base station with which communication is taking place, the path of the mobile device often results in travel toward second, third and possibly additional base stations which may be substantially equidistant from the mobile device at the time handoff is required. A handoff choice must then be made as to one of these base stations though insufficient information is available at the time handoff is required to determine which of these available and selected candidates for handoff is the correct or best available candidate for handoff. Even if the mobile device makes a turn toward one of the second, third and possibly additional base stations at the time handoff is required, which would have resulted in handoff to that one station, the network often does not become aware of this change of direction until long after handoff has been made. In addition, handoff is generally made to the candidate providing the strongest signal at that point in time, though that candidate may ultimately not be the best candidate due to the new direction being taken by the mobile device. When the handoff is not to the best base station, a further handoff is then required which should have been unnecessary, thereby unnecessarily tying up at least one of the base stations and losing a portion of its service (capacity) for a period of time. In addition, excess network traffic is incurred along with the possibility of a dropped call increasing with each handoff.
Handoff algorithms using pattern recognition are presently being developed wherein the base station collects received signal level information for the mobile device as a function of time (or distance, if equipped with the appropriate sensors) which varies with fading, shadowing and the essentially constant signal affecting environment. Using previously stored data, the base station employs pattern recognition to determine to which other base station the mobile device will be connected after handoff. Each micro-cell or base station has a database of signal level patterns and compares these stored patterns with the incoming information from the mobile device. When the mobile device approaches a pattern whereby it is known that handoff to another base station should soon take place, the network is notified and handoff to the appropriate new base station is assisted by this information. It is assumed that the environment (reflections from buildings and other objects) does not change with time. This type of system also suffers from the same handoff problem, to the wrong base station, as set forth above.
It is therefore apparent that a handoff technique is desirable which can determine the direction of the mobile device immediately prior to handoff so that handoff can take place with the most desirable base station out of a plurality of possible base station candidates for handoff.
In accordance with the present invention, the above described problem inherent in the prior art is minimized and there is provided a system for handoff which determines the most favorable base station to which handoff should be made prior to handoff.
Briefly, the above is accomplished by determining if a change of direction of the mobile device is taking place as well as the direction of the change of the mobile device. This is accomplished by, for example, a micro-machined gyroscope, an accelerometer, an electronic compass, devices determining direction based upon the magnetic field of the earth, and the like. These are techniques which work in a practical manner in the urban environment. The Global Positioning System (GPS) does not accurately determine heading or position in the urban, street-level, environment. The directional information is fed back to the base station and allows the base station to determine which one of plural candidates for handoff should be selected. In accordance with the present invention, ambiguity in the multi-base station environment is removed. The network switching traffic is lowered dramatically by directly choosing the correct new base station from the measurement of the direction of the mobile device and the pattern recognition at the original base station. The invention is accomplished by providing a micro-machined gyroscope internal to the mobile device to provide turning information to the network. A three dimensional capacitive-based accelerometer sensor can provide the velocity information to discern relative distance information for the basic pattern generation.